In a variety of telecommunications and other applications, batteries [e.g., valve-regulated lead acid (VRLA) batteries] are employed to provide reserve energy to the equipment powered thereby. With the increasing trend toward distributed power systems, battery reserve systems are often located in outdoor uncontrolled environments. Over a decade of experience in using VRLA batteries in outdoor environments has clearly shown that high temperatures drastically reduce the life of the battery. The lifetime of a typical VRLA battery with a rated life of ten years at a constant operating ambient temperature of 25.degree. C. will be reduced by a factor of two for approximately every 7.degree. C. rise in average operating temperature. When deployed in outdoor environments, the batteries are generally placed in closed cabinets with poor heat-exchange characteristics. The batteries are, therefore, exposed to high temperatures with poor ventilation. As a result, a ten-year rated battery may have its lifetime reduced to a quarter or a third of its rated value, especially in warmer climates such as Dallas, Texas.
While reducing the temperature of the operating environment of the battery is an important factor in sustaining the life of the battery, there are other ancillary considerations as well. The system employed to maintain the battery in a state of readiness (i.e., fully charged) is another important consideration in battery reserve systems. A known technique to improve the life of a battery is to employ an intermittent charging system. An intermittent charging system is disclosed in A New Concept: Intermittent Charging of Lead Acid Batteries in Telecommunication Systems, by D. P. Reid, et al. (Reid), Proceedings of INTELEC 1984, pp. 67-71, which is incorporated herein by reference.
Since the commercial AC power source is typically available about 99.9% of the time, the battery is conventionally maintained in a float mode whereby the battery is fully charged and is essentially being topped-off continuously. With an intermittent charging system, the battery is only charged a fraction of the time and, otherwise, the battery is disconnected from the charging circuit. Such a system is very sensible with VRLA batteries especially in view of the fact that VRLA batteries suffer from relatively low self-discharge rates (e.g., less than 10% over a 180 day period at about 25.degree. C.). Analogous to the loss of battery capacity at higher temperatures, it is estimated that the self-discharge rate approximately doubles for every 10.degree. C. rise in temperature. Even with the increase in self-discharge rates associated with higher operating temperatures, a relatively low duty cycle (i.e., ratio of the charging time to total time) is sufficient to maintain the battery in a state of readiness should the commercial power source be interrupted.
Since elements of the battery experience aging during float charging (e.g., excess current contributes to grid corrosion of the positive plate of the battery and water loss), it would be advantageous to decrease the period of time that the battery is in the float mode. As disclosed in Reid, the life of a battery may double by employing a 50% float duty cycle over a full float duty cycle operation for a particular battery design. Therefore, a reduction in the float mode duty cycle significantly increases the life of the battery.
As previously mentioned, Reid discloses a system that intermittently charges the battery. Reid fails to recognize, however, that float charging contributes to excess current charging of the battery (in excess of the charge necessary to compensate for the charge being replenished or lost during self-discharge) thereby unnecessarily heating the battery. The reactions that diminish battery life during float charging are accelerated at higher temperatures thereby further contributing to the degradation of the life of the battery.
Accordingly, what is needed in the art is a recognition that charging a battery under particular environmental conditions affects the life of the battery and, more particularly, what is needed is an intermittent system for charging a battery that overcomes the deficiencies in the prior art.